JP2011094015A - Rubber composition for tread and pneumatic tire - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a rubber composition for tires, which can prevent the reversion of vulcanization, can improve low fuel cost, grip performance, and abrasion resistance in good balance, and is excellent in processability, and to provide a pneumatic tire using the rubber composition. <P>SOLUTION: The rubber composition for treads comprises a rubber component, an aromatic vinyl polymer having a glass transition temperature of ≤10°C and having an aromatic vinyl monomer unit content of ≥95 mass%, and a mixture of a zinc aliphatic carboxylate and a zinc aromatic carboxylate. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

The present invention relates to a rubber composition for a tread and a pneumatic tire using the same.

Conventionally, in rubber compositions for tires, synthetic rubbers such as natural rubber and butadiene exhibiting excellent tear strength have been widely used as rubber components. When natural rubber is used in the rubber composition, a phenomenon called reversion occurs in which the rubber deteriorates or the cross-linked state deteriorates due to sulfur vulcanization.

In recent years, tires are often produced by vulcanization at a high temperature for a short time in order to increase the productivity of tires. In such a case, the above phenomenon is particularly remarkable. Furthermore, the modulus and hardness may decrease due to reversion, or tan δ may increase unnecessarily, resulting in a deterioration in fuel consumption.

In Patent Document 1, a mixture of an aliphatic carboxylic acid and an aromatic carboxylic acid zinc salt is used to improve rolling resistance, workability, wear resistance, and wet skid performance while suppressing reversion. Proposed. Patent Document 2 discloses a method of blending a polymer obtained by polymerizing only an aromatic vinyl monomer as a method for improving grip performance and wear resistance.

However, it is difficult to improve fuel economy, grip performance, and wear resistance in a well-balanced manner while suppressing reversion, and there is still room for improvement in this regard.

JP 2007-321041 A JP 2007-112994 A

The present invention solves the above problems and suppresses vulcanization reversion, while improving fuel economy, grip performance, and wear resistance in a well-balanced manner, and a rubber composition for a tire excellent in processability, and An object is to provide a used pneumatic tire.

The present invention relates to a rubber component, an aromatic vinyl polymer having a glass transition temperature of 10 ° C. or less and an aromatic vinyl monomer unit content of 95% by mass or more, a zinc salt of an aliphatic carboxylic acid, and The present invention relates to a rubber composition for a tread containing a mixture of zinc salts of aromatic carboxylic acids.

In the rubber composition, with respect to 100 parts by mass of the rubber component, 5 to 150 parts by mass of the aromatic vinyl polymer, 1 to 3 of a mixture of the zinc salt of the aliphatic carboxylic acid and the zinc salt of the aromatic carboxylic acid It is preferable to contain 10 parts by mass.

The content of the aromatic vinyl monomer unit in the aromatic vinyl polymer is preferably 100% by mass. In this case, the aromatic vinyl polymer is preferably polystyrene.

In the rubber composition, the content of styrene butadiene rubber in the rubber component is preferably 30% by mass or more.
The present invention also relates to a pneumatic tire having a tread produced using the rubber composition.

According to the present invention, an aromatic vinyl polymer having a glass transition temperature of not more than a certain value and a content of aromatic vinyl monomer units being not less than a certain value, a zinc salt of an aliphatic carboxylic acid and an aromatic carboxylic acid Since the rubber composition contains a mixture of zinc salts of the above, the use of the rubber composition in the tread improves the fuel economy, grip performance, and wear resistance in a well-balanced manner while suppressing reversion. A pneumatic tire can be provided. The rubber composition also has good processability.

The rubber composition for a tread of the present invention comprises an aromatic vinyl polymer having a glass transition temperature of not more than a certain value and an aromatic vinyl monomer unit content not less than a certain value, a zinc salt of an aliphatic carboxylic acid, and And a mixture of zinc salts of aromatic carboxylic acids. By blending the above mixture, vulcanization reversion can be suppressed and rolling resistance can be reduced. Furthermore, by using the specific aromatic vinyl polymer in combination, tan δ can be increased to improve grip performance, and wear resistance can also be improved. And since the above effect expresses synergistically, it becomes possible to improve these performances in a very balanced manner.

Furthermore, since reversion can be suppressed even by vulcanization at a high temperature for a short time, productivity can be improved and good processability can be obtained in an unvulcanized rubber composition.

The rubber component used in the rubber composition of the present invention is not particularly limited. For example, natural rubber (NR), styrene butadiene rubber (SBR), butadiene rubber (BR), butyl rubber (IIR), halogenated butyl rubber (X- A diene rubber such as IIR) can be used. Especially, it is preferable to use SBR and NR, and it is more preferable to use SBR and NR together because rolling resistance and grip performance are advantageous.

Examples of SBR include emulsion polymerization SBR (E-SBR) and solution polymerization SBR (S-SBR), but the degree of freedom of styrene and vinyl is high, and it is easy to design so that the performance such as grip performance can be sufficiently improved. For this reason, S-SBR is preferable.

As the S-SBR, those having a high molecular weight coupled with Sn or Si can be used. The coupling method of S-SBR is, for example, by reacting an alkali metal (such as Li) or alkaline earth metal (such as Mg) at the molecular chain terminal of S-SBR with tin halide, silicon halide or the like. Can be obtained. Moreover, S-SBR coupled with an epoxy group can also be used.

Moreover, as S-SBR, what modified | denatured the polymerization terminal and / or main chain of the said rubber | gum (The functional group which can react with a silanol group is introduce | transduced by modification | denaturation) can also be used. For example, end-modified by reacting an alkali metal or alkaline earth metal at the polymerization end of rubber with various compounds such as amides, ureas, lactams (for example, introducing epoxy groups, amino groups, etc. at the ends) Are).

The amount of styrene unit in S-SBR is preferably 5% by mass or more, more preferably 15% by mass or more, and still more preferably 20% by mass or more. The styrene unit amount is preferably 45% by mass or less, more preferably 35% by mass or less, and still more preferably 30% by mass or less.

The vinyl unit amount of S-SBR is preferably 20% by mass or more, more preferably 50% by mass or more. The vinyl unit amount is preferably 70% by mass or less, more preferably 65% by mass or less.

When the styrene unit amount and vinyl unit amount of S-SBR are within the above ranges, rolling resistance can be reduced and good wear resistance can be obtained. Also, excellent grip performance can be obtained.
The styrene unit amount and the vinyl unit amount can be calculated by H ¹ -NMR measurement.

The content of SBR in 100% by mass of the rubber component is preferably 30% by mass or more, more preferably 50% by mass or more, and further preferably 60% by mass or more. If it is less than 30% by mass, grip performance tends to be inferior. Further, the content of SBR in 100% by mass of the rubber component is preferably 95% by mass or less, more preferably 90% by mass or less, and still more preferably 85% by mass or less. If it exceeds 95% by mass, the wear performance tends to decrease.

The NR is not particularly limited, and for example, those commonly used in the tire industry such as SIR20, RSS # 3, TSR20, and the like can be used.

The content of NR in 100% by mass of the rubber component is preferably 5% by mass or more, more preferably 10% by mass or more, and further preferably 15% by mass or more. If it is less than 5% by mass, the fracture performance tends to be inferior. Further, the content of NR in 100% by mass of the rubber component is preferably 50% by mass or less, more preferably 30% by mass or less, and still more preferably 25% by mass or less. When it exceeds 50% by mass, grip performance tends to decrease.

The total content of SBR and NR in 100% by mass of the rubber component is preferably 75% by mass or more, more preferably 85% by mass or more, still more preferably 95% by mass or more, and most preferably 100% by mass. If it is less than 75% by mass, the processability of rubber tends to decrease.

The rubber composition of the present invention contains an aromatic vinyl polymer. In this specification, the aromatic vinyl polymer refers to those having an aromatic vinyl monomer unit content of 95% by mass or more. The aromatic vinyl polymer is not included in the rubber component.

Examples of the aromatic vinyl monomer (unit) constituting the aromatic vinyl polymer include styrene, α-methylstyrene, 1-vinylnaphthalene, 3-vinyltoluene, ethylvinylbenzene, divinylbenzene, and 4-cyclohexylstyrene. 2,4,6-trimethylstyrene and the like. These may be one type or two or more types. Among these, it is preferable that it is at least one selected from the group consisting of styrene, α-methylstyrene, and 1-vinylnaphthalene because it is economical, easy to process, and excellent in grip properties, and is styrene. Is more preferable.

The aromatic vinyl polymer may also contain components (ethylene, propylene, butadiene, isoprene, etc.) other than the aromatic vinyl monomer unit, but it is preferable to reduce the content of the components as much as possible. Olefinic monomers such as ethylene and propylene have poor compatibility with the rubber component, so that if the content of the olefinic monomer unit is increased, bleeding out tends to occur. Also, diene monomers such as butadiene and isoprene have a hysteresis loss that decreases due to co-crosslinking with the rubber component, and therefore the grip performance tends to deteriorate when the content of the diene monomer unit increases. . For these reasons, the content of the aromatic vinyl monomer unit in the aromatic vinyl polymer is 95% by mass or more, preferably 97% by mass or more, more preferably 99% by mass or more, and most preferably 100% by mass. %. That is, as the aromatic vinyl polymer, a homopolymer of an aromatic vinyl monomer is preferable, and polystyrene is particularly preferable.

The glass transition temperature (Tg) of the aromatic vinyl polymer is preferably −50 ° C. or higher, more preferably −45 ° C. or higher, and further preferably −40 ° C. or higher. When the temperature is lower than −50 ° C., the hysteresis loss in the rubber is small and the grip performance tends to be lowered. The Tg of the aromatic vinyl polymer is 10 ° C. or lower, preferably 5 ° C. or lower, more preferably 0 ° C. or lower. When it exceeds 10 ° C., wear resistance and grip performance at low temperatures tend to be lowered.
In this specification, Tg is a value measured according to JIS-K7121, using an automatic differential scanning calorimeter (DSC-60A) manufactured by Shimadzu Corporation under the condition of a temperature rising rate of 10 ° C./min. is there.

The weight average molecular weight (Mw) of the aromatic vinyl polymer is preferably 300 or more, more preferably 350 or more. If it is less than 300, there is a tendency that it is difficult to obtain a sufficient improvement effect of rolling resistance characteristics and grip performance. The weight average molecular weight of the aromatic vinyl polymer is preferably 20000 or less, more preferably 15000 or less. When it exceeds 20000, the rolling resistance characteristic tends to be lowered.
In this specification, the weight average molecular weight is determined by gel permeation chromatograph (GPC) (GPC-8000 series, manufactured by Tosoh Corporation), detector: differential refractometer, column: TSKGEL SUPERMALTPORE HZ- manufactured by Tosoh Corporation. It is determined by standard polystyrene conversion based on the measured value by M).

The content of the aromatic vinyl polymer is preferably 5 parts by mass or more, more preferably 10 parts by mass or more with respect to 100 parts by mass of the rubber component. If it is less than 5 parts by mass, the grip performance improving effect tends to be difficult to obtain. The content of the aromatic vinyl polymer is preferably 150 parts by mass or less, more preferably 100 parts by mass or less, still more preferably 50 parts by mass or less, particularly preferably 20 parts by mass with respect to 100 parts by mass of the rubber component. It is as follows. When it exceeds 150 mass parts, there exists a tendency for abrasion resistance to fall.

In the present invention, a mixture of a zinc salt of an aliphatic carboxylic acid and a zinc salt of an aromatic carboxylic acid is used. Thereby, reversion of NR can be suppressed, and a rubber composition having low rolling resistance and good wear resistance can be produced. Moreover, high rigidity is obtained and excellent steering stability is obtained. In particular, the deterioration of these characteristics when vulcanized at a high temperature can be suppressed. Furthermore, when the mixture is used in combination with the aromatic vinyl polymer, the effects of the present invention can be obtained synergistically, and the above-described effects can be obtained in a well-balanced manner.

As the aliphatic carboxylic acid in the zinc salt of aliphatic carboxylic acid, palm oil, palm kernel oil, camellia oil, olive oil, almond oil, canola oil, peanut oil, rice sugar oil, cocoa butter, palm oil, soybean oil, Examples include aliphatic carboxylic acids derived from vegetable oils such as cottonseed oil, sesame oil, linseed oil, castor oil and rapeseed oil, aliphatic carboxylic acids derived from animal oils such as beef tallow, and aliphatic carboxylic acids chemically synthesized from petroleum. It is also possible to prepare for future reductions in the supply of petroleum, and furthermore, since vulcanization reversion can be sufficiently suppressed, aliphatic carboxylic acids derived from vegetable oils are preferred, and palm oil, palm kernel oil or palm Oil-derived aliphatic carboxylic acids are more preferred.

The aliphatic carboxylic acid preferably has 4 or more carbon atoms, more preferably 6 or more carbon atoms. If the aliphatic carboxylic acid has less than 4 carbon atoms, the dispersibility tends to deteriorate. The carbon number of the aliphatic carboxylic acid is preferably 16 or less, more preferably 14 or less, and still more preferably 12 or less. When the carbon number of the aliphatic carboxylic acid exceeds 16, there is a tendency that the vulcanization return cannot be sufficiently suppressed.

The aliphatic group in the aliphatic carboxylic acid may be a chain structure such as an alkyl group or a cyclic structure such as a cycloalkyl group.

Examples of the aromatic carboxylic acid in the zinc salt of the aromatic carboxylic acid include benzoic acid, phthalic acid, melittic acid, hemimellitic acid, trimellitic acid, diphenic acid, toluic acid, and naphthoic acid. Of these, benzoic acid, phthalic acid, or naphthoic acid is preferable because reversion can be sufficiently suppressed.

The content ratio of the zinc salt of aliphatic carboxylic acid and the zinc salt of aromatic carboxylic acid in the mixture (molar ratio, zinc salt of aliphatic carboxylic acid / zinc salt of aromatic carboxylic acid, hereinafter referred to as the content ratio) is 1/20 or more is preferable, 1/15 or more is more preferable, and 1/10 or more is still more preferable. If the content ratio is less than 1/20, it is not possible to consider the environment or prepare for a future reduction in the amount of oil supplied, and the dispersibility and stability of the mixture tend to deteriorate. The content ratio is preferably 20/1 or less, more preferably 15/1 or less, and still more preferably 10/1 or less. When the content ratio exceeds 20/1, there is a tendency that the vulcanization return cannot be sufficiently suppressed.

The zinc content in the mixture is preferably 3% by mass or more, and more preferably 5% by mass or more. If the zinc content in the mixture is less than 3% by mass, there is a tendency that the vulcanization return cannot be sufficiently suppressed. Moreover, 30 mass% or less is preferable and, as for the zinc content rate in a mixture, 25 mass% or less is more preferable. When the zinc content in the mixture exceeds 30% by mass, the workability tends to decrease and the cost increases unnecessarily.

The content of the mixture is preferably 1 part by mass or more, more preferably 2 parts by mass or more with respect to 100 parts by mass of the rubber component. If it is less than 1 part by mass, sufficient anti-vulcanization resistance cannot be ensured, and improvement effects such as a rolling resistance reduction effect tend to be difficult to obtain. The content of the mixture is preferably 10 parts by mass or less, more preferably 8 parts by mass or less, and still more preferably 5 parts by mass or less. When the amount exceeds 10 parts by mass, the tendency to bloom increases and the viscosity decreases unnecessarily and the workability tends to deteriorate.

In the present invention, it is preferable to further add a reinforcing filler in addition to the aromatic vinyl polymer and the mixture. The reinforcing filler is not particularly limited as long as it is conventionally used in rubber compositions for tires, such as carbon black, silica, calcium carbonate, magnesium carbonate, clay, alumina, talc, etc. It is preferable to use it. These reinforcing fillers may be used alone or in combination of two or more.

Examples of carbon black that can be used in the rubber composition of the present invention include HAF, ISAF, and SAF, but are not particularly limited.

Carbon black having an average particle size of 31 nm or less and / or a DBP oil absorption of 100 ml / 100 g or more is preferable. If the viscosity is too low, the unvulcanized rubber composition becomes difficult to handle, and the molded products are excessively adhered to each other, and the moldability is deteriorated or the workability is impaired. When used with the above mixture, the viscosity of the unvulcanized rubber can be increased and the processability can be improved. Also, by blending such carbon black, block rigidity, uneven wear resistance, wear resistance, and breaking strength can be ensured.

If the average particle size of the carbon black exceeds 31 nm, the reinforcing property tends to deteriorate. The average particle diameter is more preferably 25 nm or less, still more preferably 23 nm or less. The average particle diameter is preferably 15 nm or more, more preferably 19 nm or more. If it is less than 15 nm, the workability deteriorates, the dispersion becomes worse, and the cost tends to increase.
In the present invention, the average particle diameter is a number average particle diameter and is measured by a transmission electron microscope.

If the DBP oil absorption of carbon black is less than 100 ml / 100 g, good fuel efficiency tends to be not obtained. The DBP oil absorption is more preferably 105 ml / 100 g or more, and still more preferably 110 ml / 100 g or more. The DBP oil absorption is preferably 160 ml / 100 g or less, more preferably 130 ml / 100 g or less. When it exceeds 160 ml / 100 g, there is a tendency that good fracture characteristics cannot be obtained.

The nitrogen adsorption specific surface area (N ₂ SA) of carbon black is preferably 50 m ² / g or more, more preferably 100 m ² / g or more. When it is less than 50 m ² / g, the reinforcing property is lowered and the fracture strength tends to be deteriorated. Also, _N 2 SA of carbon black is preferably 200 meters ² / g or less, and more preferably not more than 150m ² / g. When it exceeds 200 m ² / g, the low heat build-up of the rubber composition after vulcanization is inferior, and the fuel efficiency tends to deteriorate. The nitrogen adsorption specific surface area of carbon black is determined by the method A of JIS K6217, item 7.

The content of carbon black is preferably 10 parts by mass or more, more preferably 20 parts by mass or more, and still more preferably 30 parts by mass or more with respect to 100 parts by mass of the rubber component. If it is less than 10 parts by mass, the reinforcing property is insufficient, and it tends to be difficult to ensure the necessary block rigidity, steering stability, uneven wear resistance, and fracture strength. The carbon black content is preferably 100 parts by mass or less, more preferably 80 parts by mass or less, and still more preferably 60 parts by mass or less. If it exceeds 100 parts by mass, the workability tends to deteriorate or the hardness tends to be too high.

In addition to the above components, the rubber composition of the present invention includes oil, zinc oxide, tackifier, antioxidant, ozone degradation inhibitor, anti-aging agent, vulcanizer, vulcanization accelerator, vulcanization accelerator, and the like. In addition, additives as necessary can be appropriately blended.

The rubber composition of the present invention is produced by a general method. That is, it can be produced by a method of kneading each of the above components with a kneader such as a Banbury mixer, a kneader, or an open roll, and then vulcanizing.

The rubber composition of the present invention is used as a tread for a pneumatic tire.

The pneumatic tire of the present invention is produced by a usual method using the rubber composition.
That is, the rubber composition containing the above components is extruded in accordance with the shape of the tread at an unvulcanized stage and molded together with other tire members on a tire molding machine by a normal method. Form a vulcanized tire. The pneumatic tire of the present invention can be manufactured by heating and pressurizing this unvulcanized tire in a vulcanizer.

The pneumatic tire can be suitably used for heavy-duty vehicles such as passenger cars, trucks and buses.

The present invention will be specifically described based on examples, but the present invention is not limited to these examples.

Hereinafter, various chemicals used in Examples and Comparative Examples will be described.
NR: RSS # 3 (Tech Bee Hang)
SBR: E15 manufactured by Asahi Kasei Chemicals Corporation (S-SBR coupled with an epoxy group, styrene unit amount: 23% by mass, vinyl unit amount: 64% by mass, end group: none)
Carbon black: Diamond black N220 (N ₂ SA: 114 m ² / g, average particle size: 20 nm, DBP oil absorption: 114 ml / 100 g) manufactured by Mitsubishi Chemical Corporation
Aroma oil: Diana Process AH-24 (manufactured by Idemitsu Kosan Co., Ltd.)
Mixture (mixture of zinc salt of aliphatic carboxylic acid and zinc salt of aromatic carboxylic acid): Activator 73A ((i) aliphatic carboxylic acid zinc salt: fatty acid derived from palm oil (carbon number: carbon number) 8-12) zinc salt, (ii) aromatic carboxylic acid zinc salt: zinc benzoate, content molar ratio: 1/1, zinc content: 17% by mass)
Aromatic vinyl polymer (1): synthesized in the following Production Example 1 (aromatic vinyl monomer unit: styrene, styrene content: 100% by mass, glass transition temperature: −20 ° C., weight average molecular weight: 830)
Aromatic vinyl polymer (2): synthesized in Production Example 2 below (aromatic vinyl monomer unit: styrene, styrene content: 100% by mass, glass transition temperature: 0 ° C., weight average molecular weight: 1020)
Aromatic vinyl polymer (3): synthesized in the following Production Example 3 (aromatic vinyl monomer unit: 1-vinylnaphthalene, 1-vinylnaphthalene content: 100 mass%, glass transition temperature: 0 ° C., weight average molecular weight : 860)
Aromatic vinyl polymer (4): synthesized in the following Production Example 4 (aromatic vinyl monomer unit: styrene, styrene content: 100 mass%, glass transition temperature: 50 ° C., weight average molecular weight: 3130)
Zinc oxide: Zinc oxide (Mitsui Metal Mining Co., Ltd.)
Sulfur: Powdered sulfur (manufactured by Karuizawa Sulfur Co., Ltd.)
Vulcanization accelerator: Noxeller CZ (Ouchi Shinsei Chemical Co., Ltd.)

Production Example 1 (Synthesis of aromatic vinyl polymer (1))
In a 50 ml container thoroughly purged with nitrogen, 35 ml of hexane, 5 ml of styrene and 2.7 ml of a 1.6 mol / l n-butyllithium hexane solution are mixed, stirred at room temperature for 1 hour, and then reacted by adding methanol. The aromatic vinyl polymer (1) was synthesized by stopping.

Production Example 2 (Synthesis of aromatic vinyl polymer (2))
In a 50 ml container thoroughly purged with nitrogen, 35 ml of hexane, 5 ml of styrene, and 3.4 ml of a 1.6 mol / l n-butyllithium hexane solution are mixed, stirred at room temperature for 1 hour, and then reacted by adding methanol. The aromatic vinyl polymer (2) was synthesized by stopping.

Production Example 3 (Synthesis of aromatic vinyl polymer (3))
In a 50 ml container thoroughly purged with nitrogen, 35 ml of hexane, 5 ml of 1-vinylnaphthalene and 2.3 ml of 1.6 mol / l n-butyllithium hexane solution are mixed, stirred for 1 hour at room temperature, and then added with methanol. Then, the reaction was stopped to synthesize an aromatic vinyl polymer (3).

Production Example 4 (Synthesis of aromatic vinyl polymer (4))
In a 50 ml container fully purged with nitrogen, 35 ml of hexane, 5 ml of styrene, and 0.9 ml of a 1.6 mol / l n-butyllithium hexane solution are mixed, stirred at room temperature for 1 hour, and then reacted by adding methanol. The aromatic vinyl polymer (4) was synthesized by stopping.

Examples 1-6 and Comparative Examples 1-5
According to the contents shown in Table 1, materials other than sulfur and a vulcanization accelerator were kneaded for 5 minutes at 150 ° C. using a Banbury mixer to obtain a kneaded product. Next, using a biaxial open roll, sulfur and a vulcanization accelerator were added to the obtained kneaded product, and kneaded for 5 minutes at 50 ° C. to obtain an unvulcanized rubber composition.
The obtained unvulcanized rubber composition was press vulcanized under the vulcanization conditions shown in Table 1 to obtain a vulcanized rubber composition.
Further, the obtained unvulcanized rubber composition is processed into a tread shape, bonded to another tire member, and vulcanized under the vulcanization conditions shown in Table 1 (tire size: 215 / 45R17 size).

The following evaluation was performed using the obtained unvulcanized rubber composition, vulcanized rubber composition, and test tire. The results are shown in Table 1.

(1) Using a reversion rate curast meter, the vulcanization curve of the unvulcanized rubber composition at 170 ° C. was measured. The maximum torque (MH) value, the minimum torque (ML) value, and the torque value M15 15 minutes after the start of vulcanization were measured, and the vulcanization return rate of each formulation was evaluated by the following formula. The smaller the reversion rate (reversion rate), the lower the reversion and the better.
(Reversion rate (%)) = (MH−M15) / (MH−ML) × 100

(2) Workability In accordance with JIS K 6300-1 “Unvulcanized rubber—Physical characteristics—Part 1: Determination of viscosity and scorch time using Mooney viscometer”, using a Mooney viscosity tester for 1 minute Under the temperature condition of 130 ° C. heated by preheating, the small rotor was rotated, and the Mooney viscosity (ML _{1 +} 4/130 ° C.) of the unvulcanized rubber composition after 4 minutes was measured. The numbers after the decimal point are rounded off. The processability was evaluated based on the Mooney viscosity.

(3) Rolling resistance Using a rolling resistance tester, each test tire was mounted on a regular rim, and the rolling resistance was measured at an internal pressure of 200 kPa, a speed of 80 km / h, and a load of 5.0 KN. It was displayed as an index (rolling resistance index). The smaller the value, the smaller the rolling resistance and the better.

(4) Grip performance The test tire was mounted on a vehicle, and the test driver evaluated the grip performance by running 10 laps on the asphalt road test course. The grip performance of the tire of Comparative Example 1 was evaluated as 3 points, with 3 points. The grip performance of the first lap was evaluated as the initial grip performance, and the laps 2 to 10 were evaluated as the grip performance. The larger the value, the better the grip performance and the better (5: good, 4: somewhat good, 3: normal, 2: slightly bad, 1: bad).

(5) Wear resistance performance The test tire was mounted on a vehicle, the test course was run 20 laps, and the change in groove depth before and after running was measured. Using the depth of Comparative Example 1 as a reference, each index was expressed by the following formula (wear resistance index). The higher the wear resistance index, the better the wear resistance.
(Abrasion resistance index) = (Change in groove depth of Comparative Example 1) / (Change in groove depth of each formulation) × 100

From Table 1, in the examples in which a specific aromatic vinyl polymer having a Tg of 10 ° C. or less and the above mixture were blended, reversion was suppressed, and fuel economy, grip performance, and wear resistance were improved in a well-balanced manner. did it. On the other hand, in the comparative example, these performances could not be obtained with a good balance. Furthermore, when the results of Examples 1 to 4 and Comparative Examples 1 to 4 were compared, the above performance (particularly, abrasion resistance) was synergistically obtained in the Examples.

Claims (6)

A rubber component, an aromatic vinyl polymer having a glass transition temperature of 10 ° C. or less and an aromatic vinyl monomer unit content of 95% by mass or more, a zinc salt of an aliphatic carboxylic acid, and an aromatic carboxylic acid A rubber composition for treads containing a mixture of zinc salts. 5 to 150 parts by mass of the aromatic vinyl polymer and 1 to 10 parts by mass of a mixture of the aliphatic carboxylic acid zinc salt and the aromatic carboxylic acid zinc salt with respect to 100 parts by mass of the rubber component. Item 2. A rubber composition for a tread according to Item 1. The rubber composition for a tread according to claim 1 or 2, wherein the content of the aromatic vinyl monomer unit in the aromatic vinyl polymer is 100% by mass. The rubber composition for a tread according to claim 3, wherein the aromatic vinyl polymer is polystyrene. The rubber composition for a tread according to any one of claims 1 to 4, wherein the content of the styrene butadiene rubber in the rubber component is 30% by mass or more. A pneumatic tire having a tread produced using the rubber composition according to claim 1.